The present invention relates to data communication networks. In particular, the present invention relates to a procedure for ensuring emergency communication in an access network or access node connected to a data communication network and having a number of subscriber lines connected to it for the connection of subscriber
There are at least two commonly known methods for connecting subscribers to a local exchange in a data communication network. In the first method, the subscriber is connected via a subscriber line directly to the local exchange or to an access module connected to it. In the second method, the subscriber is connected to an access network, whose access node is connected to the exchange.
Open interfaces (V5.1 and V5.2) between an access network and a local exchange are defined in the ETSI (European Telecommunications and Standards Institute) standards of the ETS 300 324 and ETS 300 347 series. V5 interfaces enable subscribers belonging to a physically separate access network to be connected to a telephone exchange using a standard interface. In this application, V5 interface refers in particular to a dynamic concentrator interface (V5.2) consistent with the standards ETS 300 347-1 and 347-2, which consists of one or more (1-16) PCM (Pulse Cone Modulation) cables. One PCM cable comprises 32 channels, each of which with a transfer rate of 64 kbit/s, i.e. 2048 kbit/s in all. The V5.2 interface supports analogue telephones as used in the public telephone network, digital, such as ISDN (Integrated Services Digital Network) basic and system subscriptions as well as other analogue or digital terminal equipment based on semi-fixed connections.
The terminal equipment can be connected to the access ports of the access node. One or more V5 interfaces can be connected to he access node. The access ports are created in the V5 interface by linking an unambiguous address of each port with a given address of the V5 interface. In the local exchange, this address is created as a V5 subscriber. In other words, each access port has an address which is linked with a V5 address and uses a given time slot or given time slots for signalling to the local exchange.
The objective in ensuring emergency communication is to make sure that calls to emergency numbers of subscribers connected to an access node can be set up even when the V5 interface between the access node and the telephone exchange is out of order. The V5.x standards contain no definition of systems for ensuring emergency communication in case of failure. In this situation, the problem is that, at present, all communication, including emergency communication, via a specific V5 interface of a given subscriber is hindered when the V5 interface fails.
One possibility for ensuring emergency communication in case of failure of a V5 interface assigned to a subscriber is to create in the telephone exchange to which the V5 interface is connected a separate emergency address for each subscriber for communication during the failure situation. However, the problem with this system is the high capacity required in the exchange, because in this case the exchange must have a double subscriber address range as compared with the number of subscribers. In addition, such an arrangement would significantly increase the size of the interface database of the access node.
The object of the present invention is to present a new type of a procedure and system for ensuring emergency communication in the event of a failure of the V5 interface between an access node and a local exchange. A further object of the present invention is to present a system which does not require an oversized capacity in the exchange connected to the access node.
As for the features characteristic of the invention, reference is made to the claims.
In the procedure of the present invention for ensuring emergency communication in a data communication network comprising a number of telephone exchanges interconnected via trunk cables, the data communication network comprises an access node which is connected to at least one of the exchanges. Connected to the access node, preferably via subscriber cables, are a number of terminal devices used by subscribers to set up connections over the data communication network. Further, in the procedure of the invention, signalling according to the V5 standard, which is defined in the standards mentioned above, is used in the connection between the access node and the telephone exchange, which is a V5 interface consistent with the standard.
In the procedure of the invention for ensuring emergency communication, in the access node is created a separate V5 interface with no subscribers connected to it and comprising a number of V5 subscriber addresses created in the access node. These subscriber addresses are not assigned permanently to any subscriber. The separate V5 interface with no subscribers connected to it is generally created in conjunction with the installation and start-up of the access node. The separate V5 interface is an interface that in a normal situation is not used for the switching of voice or data communication of any subscriber. In the telephone exchange to which this separate V5 interface is connected, a few permanent subscriber addresses have been created for this V5 interface, although in the access node these V5 interface addresses have not been created permanently in the subscriber-line interfaces. Further, according to the invention, in the event of failure of the V5 interface used by a subscriber, a new V5 subscriber address is allocated for the subscriber from the set of V5 subscriber addresses of the separate V5 interface. The set of subscriber. addresses of the separate V5 interface consists of a number of subscriber addresses that can be used when necessary to handle the communication of any one of the subscribers connected to the access node concerned. A failure situation occurs when a subscriber""s signalling connection to the telephone exchange fails. Further, subscriber connections are set up using a V5 subscriber address, which is released after the call is disconnected.
As compared with prior art, the present invention has the advantage that, using the procedure of the invention, emergency communication of subscribers connected to an access node can be effectively and reliably guaranteed even in the event of a failure of the V5 interface used by the subscriber. A further advantage of the invention is that it makes it unnecessary to reserve a double number of V5 addresses in the exchange to ensure successful emergency communication of V5 subscribers in a failure situation.
In an embodiment, the separate V5 interface is connected to a different telephone exchange than the other V5 interfaces of the access node. This is advantageous because in this case the connection of this interface is very likely to run along a physically different route, e.g. in a different cable trench than the other V5 interfaces of the access node. A further advantage of this arrangement is that a failure of the exchange to which the other V5 interfaces are connected is not likely to cause a failure of the separate emergency V5 interface as it is connected to a different exchange.
In a preferred embodiment of the present invention, a connection setup request given by a subscriber is monitored in the telephone exchange and, based on this monitoring, connection setup using a V5 subscriber address of the separate V5 interface is prevented if it is detected that the connection is intended for communication other than emergency communication. In a further preferred case, the first free V5 subscriber address in the set of V5 subscriber addresses of the separate V5 interface is reserved for the subscriber. This arrangement ensures that the emergency interface will not be used for communication other than emergency communication. Thus, in a failure situation, when the emergency interface is in operation, its whole capacity can be used for emergency communication. In a preferred case, the connection set up is a call set up between two terminal devices, and the emergency communication consists of an emergency call, alarm message, alarm call or the like.
According to the invention, the access node comprises a separate V5 interface having no subscribers connected to it and comprising a number of V5 subscriber addresses and means for reserving for the subscriber a new V5 subscriber address from the set of V5 subscriber addresses of the separate V5 interface in a failure situation of the V5 interface used by the subscriber. The means for reserving a new V5 subscriber address for the subscriber are preferably implemented by using the program segments of the access node. In the system of the invention, the separate V5 interface is preferably connected to a different telephone exchange than the other V5 interfaces of the access node.
In a preferred embodiment of the present invention, the system comprises means for monitoring the subscriber""s connection setup request in the telephone exchange and, based on the monitoring, the system prevents connection setup if it is detected that the connection is intended for communication other than emergency communication. The monitoring can be implemented using components existing in the telephone exchange, of which at least the push-button dialling receivers and the central control unit can be used.